In recent years, organic photoconductive materials have been mostly used as the photoconductive materials applicable to electrophotoreceptors in place of inorganic photoconductive materials. That is why various combinations of the materials and conditions for synthesis make it possible to obtain a variety of the organic photoconductive materials; the freedom for selecting the materials can be expanded; and any desired photoreceptors can be readily prepared so as to meet the purposes of the applications.
Further, the organic photoconductive photoreceptors can be of a function-separated type, in which the different materials are responsible for carrier generation and carrier transport functions, whereby the freedom for selecting the materials can be further expanded and the electrophotographic characteristics such as chargeability, sensitivity and durability have been expected to improve.
In the field of the copying industry, demands for more improved image quality and image editorial functions promote the development of the recording apparatuses such as the copiers or printers of a digital system, and the improvements in the photoreceptors therefor have been strongly desired. Generally, a photoreceptor is dotwise exposed to a laser beam modulated by an image signal to form latent dot images thereon, which are developed to visible images in a reversal development system. A desirable laser beam-emitting equipment is such that can make an exposing unit simpler, smaller-sized and less expensive and has oscillating wavelength of the laser beam in the infrared region of not shorter than 750 nm. Therefore, the photoreceptor applicable thereto is required to have a high sensitivity at least in the wavelength region of 750 to 850 nm.
A variety of organic dyes and pigments have been proposed as the carrier generation materials applicable to the foregoing function-separated type photoreceptors, and there are available in the market polycyclic quinone pigments such as dibromoanthanthrone; pyrylium dye and the eutectic complexes of the pyrylium dye and polycarbonate; squalium pigment; phthalocyanine pigment; and azo pigment. Among them, Japanese Patent Publication Open to Public Inspections (hereinafter referred to as JP OPI) No. 239248/1986, 217050/1986, 67094/1987 and 218768/1988 disclose titanyl phthalocyanine pigments having a primary sensitivity in a long wavelength region of not shorter than 750 nm. Such titanyl phthalocyanine pigments have specific coagulative structures or specific crystal forms to shift main absorption wavelengths to longer wavelength regions and make sensitivities higher. However, it is so difficult to settle the optimum conditions for preparing the above pigments that they can not totally satisfy the characteristics such as a chargeability, sensitivity and repetitive property, and a further higher sensitivity is desired.
In the ordinary type electrophotoreceptors, an electric contact of a grounded conductive layer with a photoconductive layer is not uniform from microscopic viewpoint. For example, a local difference in the carriers supplied from the conductive layer causes a local difference in the charge distribution on the surface of the photoreceptor. This results in image defects such as white spots produced on a dark background in a positive development process and dark spots produced on a white background in a negative reversal development process. In particular, the dark spots produced in the reversal development process as well as a background fog seriously spoil the quality of images. The above problems are notably caused particularly in the foregoing highly sensitized photoreceptors, and the dark spots are seriously produced in the foregoing reversal development process.
For the purpose of improving the foregoing function-separated type laminated layers, a variety of auxiliary layers has positively been studied for increasing an adhesion of a photoconductive layer to a support, protecting physically and chemically a photoconductive layer, and improving in blocking the carriers supplied from a support and transporting them from layer to layer. In addition, the numerous studies on the binders for the photoconductive layers have also been made from the similar viewpoints.
As for the binders for a subbing layer, the hydrophobicresins such as a vinyl chloride/acetate-maleic acid copolymer and an ethylene type polymer are disclosed in JP OPI No. 47047/1981 and 218658/1985, respectively, in which the adhesion properties of a support and a photoconductive layer and the effect of blocking the free carriers supplied from the support are emphasized. Further, because of the advantages of easier manufacturing, a more excellent coating property, a higher blocking efficiency of free carriers, and a higher adhesion property, water-soluble polymers have also been studied for a subbing layer. However, such water-soluble polymer has the problem that it deteriorates environmental characteristics of a photoreceptor, that is, it degrades the stability of a charge potential and a residual potential under a high temperature and humidity.
Polyvinyl butyral is known to be excellent in adhesion property, compatibility between resins, chemical resistance, both thermoplastic and thermosetting properties as well as coating property and grain dispersion property. Generally, a polyvinyl acetal resin has unsatisfactory durability in repeated use. Particularly in the operation under a high temperature and humidity, the produced carriers are affected by a moisture absorbed therein and trapped in a charge generation layer, which results in deterioration in sensitivity and increase in a residual potential, so that the various advantages of the polyvinyl butyral including the adhesion property are hindered from displaying.